It doesn't work the way you think. Just throw a bad buff on an walking NPC (Wind Walk) and see how the server and the client desynchronize.
It seems that the server is not sending information about the current speed of the NPC. At least client do not understand this. Missing packet? Unknown packet?
I don't know if it works that way in retail. Some other parameters do not work too - ground_high, ground_low (from client side), collision_height (from server side) - can't change vertical position of NPC.
Things that works from client side: drawscale (size of the NPC), npc_speed (this one change animation speed, not a speed) etc...